From the viewpoint of recent global environment, there are demands for reducing carbon dioxide emission, and therefore an increasingly high demand for reducing the weight of automobiles, construction machinery, and railway cars. Particularly, there is a strong demand for reducing weight of springs used therein, whereby high stress design is applied to these springs by using as a material thereof a strengthened material having a post quenching-tempering strength of about 1800 MPa or more.
General-purpose steel for springs has a post quenching-tempering strength of about 1600 to 1800 MPa, as prescribed in JIS G4801 and the like. Such steel for springs is manufactured into a predetermined wire rod by hot rolling, and the wire rod is thermally formed into a spring-like shape and subjected to quenching-tempering processes in the case of a hot formed spring. Alternatively, in the case of a cold formed spring, the wire rod is subjected to a drawing process and then to quenching-tempering processes and formed into a spring-like shape.
For example, in the case of a hot formed spring, the fatigue resistance of the spring is improved by applying compressive residual stress to the surface of the spring via shot peening after quenching-tempering.
The materials commonly used for the above-described springs include SUP9A described in JIS G4801. SUP9A is hot-formed into a spring-like shape, and compressive residual stress is applied to the surface via shot peening in order to improve fatigue resistance. When SUP9A, however, is manufactured into a predetermined wire rod by hot rolling, or heated for formation into a spring-like shape, the carbon in the surface layer is reduced, and total decarburization occurs. Hardness of the surface of the manufactured spring thus easily deteriorates, causing the application of compressive residual stress via shot peening to be insufficient, which adversely affects the properties of the spring (in particular, the fatigue property).
As stated above, since steel for springs is heated at least once for formation, carbon in the surface layer is reduced, and decarburization occurs. With respect to the decarburization, JIS G 0558 defines four types of decarburization layer depth: “depth of total decarburization layer”, “depth of ferrite decarburization layer”, “depth of decarburization layer with specified residual carbon ratio”, and “depth of effective decarburization”. In terms of decarburization of steel for springs, the “depth of ferrite decarburization layer” and the “depth of effective decarburization” are two problematic decarburization layer depths. The depth of ferrite decarburization layer is the depth of a layer measured from the surface of the steel material, in which layer the carbon content is nearly zero, so that despite rapid cooling after heating, the layer transforms into ferrite, and a ferrite phase forms. The depth of effective decarburization is the distance (depth) from the surface of the steel material to a position at which the carbon content is reduced compared to that of the base material, even though the carbon content does not reach zero, thereby causing hardness to deteriorate as compared to the base material in the case of rapid cooling after heating, yet an adequate hardness for practical purposes is obtained. In steel for springs, either a ferrite decarburization layer forms on the surface of the steel material with an effective decarburization occurring inwards from the ferrite decarburization layer, or depending on the chemical composition, an effective decarburization occurs without formation of a ferrite decarburization layer. As used herein, “decarburization” refers to effective decarburization. As described above, when such decarburization occurs near the steel material surface, compressive residual stress via shot peening cannot be applied sufficiently, resulting in the problem of an adverse effect on the properties of the spring, in particular the fatigue property.
A few proposals have been made to overcome this problem. JP2003105496A (PTL 1) describes high strength steel for springs that achieves low decarburization and excellent delayed fracture resistance by controlling the added amounts of C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Nb, Ti, Al, N, and B, as well as controlling the added amount of the total of As, Sn, and Sb and the added amounts of Cu and Ni. PLT 1 teaches the relationship between decarburization depth and the added amount of the total of As, Sn, and Sb, yet even optimizing the added amount of the total of As, Sn, and Sb does not manage to suppress ferrite decarburization. Therefore, PTL 1 cannot necessarily suppress the decarburization occurring inwards from the ferrite decarburization layer.
JPS61183442A (PTL 2) describes steel for springs in which decarburization is suppressed by optimizing the added amounts of C, Si, Mn, Sb, As, and Sn. PTL 2 discloses the relationship between decarburization depth and the added amounts of As, Sn, and Sb as well as the added amount of the total of As, Sn, and Sb, yet even optimizing the added amount of the total of As, Sn, and Sb does not manage to suppress ferrite decarburization. Therefore, PTL 2 cannot necessarily suppress the decarburization occurring inwards from the ferrite decarburization layer.
JPH01319650A (PTL 3) describes steel for springs in which decarburization is suppressed by optimizing the added amounts of C, Si, Mn, Cr, and Sb. As stated later, however, if Sb is added more than required, scales grow more rapidly and increases in thickness during heating of the material, thereby making it difficult to exfoliate scales during production of the material and formation of springs, and deteriorating the scale exfoliation property. Therefore, surface defects occur due to scale biting during production of the material and formation of springs, deteriorating the fatigue property of the springs.
JP2004169142A (PTL 4) describes steel for springs in which quench hardenability and pitting corrosion resistance are improved by optimizing the added amounts of C, Si, Mn, Cr, Nb, Al, N, Ti, and B and by adding Sb as the selected element. It is difficult, however, to suppress decarburization by merely adding Sb alone and, as described below, if Sb is added more than required, scales grow more rapidly and increases in thickness during heating of the material, thereby making it difficult to exfoliate scales during production of the material and formation of springs, and deteriorating the scale exfoliation property. Therefore, the material and springs suffer surface defects due to scale biting during production of the material and formation of springs, thereby deteriorating the fatigue property of the springs.
As mentioned above, since steel for springs is heated at least once, scales form on the material surface. If the added amounts of As, Sn, and Sb are within the ranges described in PTL 1 to PTL 4, scales grow more rapidly during heating of the material and increases in thickness, which prevents exfoliation of scales formed during the production of material and the formation of springs, and causes a surface indentation flaw, resulting in the problem of deteriorating the fatigue property of the resulting steel for springs. There is thus demand for steel for springs that exhibits an excellent scale exfoliation property.